1. Field Of The Invention
The present invention relates to the treatment of halogenated aromatics in hydrocarbon streams. In particular, it relates to a method for reducing the content of polychlorinated biphenyls in transformer oils containing them.
2. Description Of The Prior Art
The biodegradability of halogenated aromatic compounds, particularly chlorinated aromatics, is recognized to be related to the number of halogen atoms present in such compounds. Accordingly, it is desirable to reduce the number of halogens in such compounds to enhance their biodegradability. It is known that displacement of halogen groups in such compounds to produce their phenolic counterparts or ether precursors reduces the solubility of such compounds in organic streams and increases the solubility of such compounds in polar solvents.
Polychlorinated biphenyls or "PCB's" were long used as dielectric fluids in electrical equipment and the like because such materials possessed excellent heat stability, are non-flammable in nature, exhibit low volatility and present good viscosity characteristics at ambient operating temperatures. However, the highly chlorinated "PCB's" are almost indestructible by the environment. When present in water, they tend to concentrate in the tissue of fish, thereby producing an adverse effect on humans eating fish contaminated with such compounds. Because of their environmental persistence, the continued manufacture and importation and the new use in the United States of such PCB's was banned in 1976 and the U.S. Environmental Protection Agency promulgated rules and regulations for their removal from commerce.
As of July 1, 1979, EPA regulations defined as "PCB-contaminated" any material containing more than 50 parts per million (ppm) of a mono-, di-, or polychlorinated biphenyl. While PCB-contaminated materials can be disposed of by incineration or in a secure land fill, such procedures have rarely proven acceptable to the community at large.
Considerable fractions of transformer oils now in service are PCB-contaminated. Transformer oils include, for example, refined asphaltic-base mineral oil, and heat exchange oils, such as hydrogenated terphenyls. Accordingly, it has long been desired to provide an effective and safe method for disposing of PCB-contaminated transformer oils. As employed hereafter, the term "transformer oil" means a mineral insulating oil of petroleum origin for use as an insulating and cooling media in electrical and other apparatus.
Other halogenated aromatic hydrocarbons presenting acute environmental problems include the chlorinated aromatic dioxins, such as 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin, chlorinated aromatics, such as 2, 4, 5-trichlorophenol, DDT and polybrominated biphenyls.
Various methods have been proposed for reducing the level of halogenated aromatic hydrocarbons in hydrocarbon streams. In U.S. Pat. No. 4,326,090, halogenated organics were dehalogenated by reacting them with sodium naphthalenide in the presence of sodium metal. The preparation of sodium naphthalenide is expensive and potentially hazardous. Further, the process requires a multistep procedure involving the incorporation of the organo-sodium compound into PCB contaminated oil, followed by a minimum of a two hour reaction time, a water quench, and distillation and purification steps to recycle the potentially hazardous reaction solvent.
It has also been proposed to destroy polychlorinated biphenyls utilizing a sodium dispersion in kerosene. This reaction requires an extended heating period. The use of sodium metal necessitates the implementation of special handling procedures and the elimination of even trace amounts of water to avoid side reactions. Pytlewski, et al. of the Franklyn Research Center has reported on the reaction of PCB's with sodium, oxygen and polyethylene glycols in Treat. Haz. Waste. Symp., (1980). It was said that PCB's and other halogenated pesticides could be decomposed employing molten sodium metal dispersed in polyethylene glycols. However, the use of metallic sodium is dangerous and even trace amounts of water must be eliminated to minimize dangerous side reactions.
It has been further proposed to reduce the level of polychlorinated aromatics dissolved in organic solvents such as transformer oil, by treating the contaminated solution with a mixture of a polyethylene glycol or a derivative thereof and an alkali metal hydroxide as set forth in U.S. Pat. Nos. 4,351,718 and 4,353,793. However, it has been found that such reactions require extended periods of time to reduce the concentration of halogenated contaminants, such as PCB's, to generally acceptable levels. A similar process disclosed in U.S. Pat. No. 4,327,027 for the reduction of chlorinated aromatic dioxins is subject to essentially the same defects.
It is well known that dimethyl sulfoxide (DMSO) is a solvent which is generally immiscible in hydrocarbon oils, but is useful in extracting unsaturated compounds, such as aromatic hydrocarbns, from hydrocarbon streams. However, due to its high boiling point, DMSO is not a solvent of choice for removing halogenated aromatic contaminants to reduce their concentration to acceptably low levels.
DMSO has also been employed as a reaction solvent in displacement reactions employing an alkoxide to dehalogenate certain monohalogenated aromatic compounds also containing certain activating groups. Monohaloaromatics containing such activating groups as nitro, keto, hydroxy or the like are said to dehalogenate when sufficiently heated with an alkoxide dissolved in DSMO. Tetrahedron Letters, No. 20, pp. 1725-1728 (1970) and United Kingdom Pat. Nos. 1,143,170 and 1,316,277. Similarly, monohalonaphthalenes were said to dehalogenate using potassium butoxide in DMSO at elevated temperatures. J. Org. Chem., Vol. 36, pp. 314-322 (1971 ).
Polyhalogenated aromatics on the other hand have been disclosed to resist dehalogenation except with highly dangerous strong bases, such as alkali metals, organo metallics, or alkali amides. Recently, it has been proposed to employ alkoxides and hydroxides in polyethylene glycols to dehalogenate polyhalogenated aromatics. However, as previously discussed, such processes were found to require many hours to reduce the levels of polyhalogenated contaminants to acceptable levels.
Many groups have advocated a massive effort to eliminate polyhalogenated aromatics from the environment. However, conventional dehalogenation processes simply have not met with wide public acceptance owing, in part, to their potential hazard and undue expense.